It is desirable in many different situations to selectively transfer heat between a fluid and a body to be cooled or warmed. As used herein, the term “body” refers not only to solid bodies but also other fluid materials which take the shape of the container in which they exist.
One well known system for achieving such transfer of heat achieves cooling of a body by first pressurizing a vapor phase heat transfer fluid and then expanding it through a Joule-Thomson expansion element, such as a valve, orifice, or other type of flow constriction. Any such device will be referred to hereinafter simply as a Joule-Thompson “expansion element,” and systems which use such an element are sometimes referred to herein as Joule-Thompson systems. In most Joule-Thomson systems, single component, non-ideal gasses are pressurized and then expanded through a throttling component or expansion element, to produce isenthalpic cooling. The characteristics of the gas used, such as boiling point, inversion temperature, critical temperature, and critical pressure effect the starting pressure needed to reach a desired cooling temperature. While such characteristics are all generally well known and/or relatively easy to predict with an acceptable degree of certainty for single component fluids, this is not necessarily the case for multi-component fluids
Because of the large number of properties or characteristics which are relevant to the effectiveness and desirability of a heat transfer fluid, it is frequently difficult to predict in advance how any particular multi-component fluid will perform as a heat transfer fluid. For example, U.S. Pat. No. 5,744,052—Bivens discloses a combination of difluoromethane (HFC-32), pentafluoroethane and a small amount (ie., up to 5% by weight) of carbon dioxide in the form of an azeotropic fluid that is said to have advantages as a refrigerant in certain applications. More particularly, the multi-component fluid of Bivens is said to be non-flammable and, due to its azeotropic nature, to undergo relatively little fractionation upon vaporization. However, the combination of components in the amounts specified in Bivens produce a fluid with a relatively low vapor pressure, which can be undesirable for certain applications, for example, those which require a fluid with substantial cooling power or those in which low temperature cooling is required. Furthermore, the fluids of Bivens are comprised of relatively highly-fluorinated compounds which are potentially environmentally damaging from a global warming perspective. In addition, obtaining fluids with azeotropic properties can sometimes add significantly to the cost of such fluids when used as refrigerants.
U.S. Pat. No. 5,736,063—Richard et al. discloses a non-azeotropic combination of various hydrocarbons, including HFC-32, and carbon dioxide which form a fluid said to be acceptable as replacements for chlorodifluoromethane (HCFC-22). In particular, the Richard et al. patent teaches that the vapor pressure of this fluid is substantially equal to HCFC-22, which is only about 83 psia at 40° F. Therefore, while the fluid of Richard et al. is expected to perform well in certain refrigeration applications, it may be considered inadequate in the same types of applications mentioned above with respect to the Bivens fluid.